Emergence of a lithium dip in ~35 Myr "Snake" Open Clusters

We report the discovery of a lithium dip (Li-dip) in the stellar “Snake” (age = $35 \pm 5$ Myr), challenging the classical view that Li-dips emerge only at ages $\gtrsim 150$ Myr. Using high-resolution spectra from GALAH DR4 ($R \sim 28,000$) for 211 member stars, we identify a clear depletion feature in a $T_{\mathrm{eff}}$ range of 6200–6800 K with a depth of $ΔA(\mathrm{Li}) \approx 0.40$ dex. Our analysis reveals two key advances: the Li-dip appears $\gtrsim 100$ Myr earlier than the previous observations, and within the dip temperature range, a significant correlation is found between rotational velocity and lithium depletion. Specifically, fast rotators ($v \sin i > 25$ km s$^{-1}$) exhibit stronger lithium depletion than slow rotators ($v \sin i < 25$ km s$^{-1}$). This trend suggests that faster rotators develop stronger rotational shear at the convective-radiative boundary, which enhances turbulent mixing and accelerates lithium destruction. It is also found that the lower temperature edge of the lithium plateau can reach as low as 5500 K for the young open clusters.

💡 Research Summary

The authors present a comprehensive spectroscopic study of the young stellar association known as “Snake,” whose members share a common age of 35 ± 5 Myr and a near‑solar metallicity. Using high‑resolution (R ≈ 28,000) spectra from the GALAH Data Release 4, they analyze 211 confirmed members that satisfy strict signal‑to‑noise, temperature, and metallicity criteria. Although GALAH DR4 already provides lithium abundances derived via a neural‑network pipeline, the authors re‑measure A(Li) for all stars through a dedicated spectral synthesis approach (IDL‑based SIU) employing 1‑D LTE MARCS model atmospheres with NLTE corrections. This independent analysis yields lithium abundances that are systematically higher than the GALAH pipeline values, especially for stars with projected rotational velocities (v sin i) greater than 40 km s⁻¹, underscoring the limitations of automated pipelines for broadened lines.

The central result is the clear detection of a lithium dip (Li‑dip) in the effective temperature range 6200–6800 K. The dip reaches a depth of ΔA(Li) ≈ 0.40 dex relative to the presumed primordial lithium abundance, with a minimum A(Li) ≈ 2.80 dex near 6500 K. This feature is observed in a cluster that is roughly a factor of four younger than the youngest open clusters previously known to host a Li‑dip (e.g., the Pleiades at ~125 Myr). Consequently, the authors argue that the Li‑dip can emerge as early as ~35 Myr, i.e., about 100 Myr earlier than the conventional age threshold of ≳150 Myr.

A second, equally important finding is a statistically significant anti‑correlation between lithium abundance and stellar rotation within the dip temperature range. By dividing the dip stars into “fast rotators” (v sin i > 25 km s⁻¹) and “slow rotators” (v sin i ≤ 25 km s⁻¹), they find mean lithium abundances of ⟨A(Li)⟩ = 3.02 ± 0.03 dex for the fast group and ⟨A(Li)⟩ = 3.08 ± 0.02 dex for the slow group, a difference of 0.06 dex that exceeds the typical measurement uncertainty (≈0.1 dex). This trend supports theoretical predictions that rapid rotation enhances meridional circulation and shear‑driven turbulence at the convective‑radiative boundary, thereby transporting surface lithium to hotter interior layers where it is efficiently destroyed. The authors cite classic work by Charbonneau & Michaud (1988) and recent models by Li et al. (2025) that explicitly link rotational energy to the efficiency of large‑scale circulation and mixing.

Beyond the dip, the study confirms the presence of a lithium “plateau” for stars cooler than ~6200 K, with a mean A(Li) ≈ 3.10 dex and a remarkably small scatter (σ ≈ 0.07 dex). Notably, the lower temperature edge of this plateau extends down to ~5500 K in the Snake association, whereas older clusters such as Praesepe or Hyades show the plateau truncated near 6000 K. This suggests that the temperature extent of the lithium plateau is age‑dependent, possibly reflecting the deepening of convective envelopes and the progressive depletion of lithium in cooler stars as they evolve.

The paper also includes a careful validation of the GALAH v sin i values by independent line‑broadening measurements, finding good agreement, and a discussion of systematic differences between GALAH DR3 and DR4 lithium abundances. The authors argue that their manual synthesis approach yields more reliable lithium measurements, especially for rapid rotators where line blending and continuum placement are challenging.

In the discussion, the authors emphasize three major implications: (1) the early appearance of the Li‑dip challenges standard stellar evolution models that predict negligible lithium burning in F‑type stars at such young ages; (2) the observed rotation‑lithium correlation provides direct empirical support for rotationally induced mixing as a key non‑standard process; and (3) the age‑dependent shift of the lithium plateau’s lower boundary offers a new diagnostic for testing models of pre‑main‑sequence lithium depletion and convective envelope evolution.

Overall, this work delivers a robust observational benchmark for young, co‑eval stellar populations, demonstrating that lithium can serve as a sensitive probe of internal mixing processes already within the first 40 Myr of stellar life. The findings call for updated stellar evolution calculations that incorporate early‑time rotational shear and meridional flows, and they motivate further high‑resolution spectroscopic surveys of other young associations to test the universality of the early Li‑dip phenomenon.